The preferred embodiment of this invention is a modification or improvement over the invention disclosed in U.S. Pat. No. 5,248,560 to Baker et al, which discloses a developer roller of a metal shaft with outer roller material of metal-salt-filled urethane. That urethane is produced from polyester toluene diisocyanate and the metal salts specifically disclosed are copper (II) chloride and lithium chloride.
The materials used for rollers in the electrophotographic process must have specific electrical properties. The electrical resistivity typically must be in the range of 1.times.10.sup.7 (one times 10 to the 7th power) to 1.times.10.sup.13 ohm-cm, which is semiconductive. Polyurethane has resistivities of 1.times.10.sup.10 to 1.times.10.sup.15 ohm-cm. Therefore, conductive additives must be used to reduce the electrical resistivity to the desired value. Metal halides are commonly used as conductive additives. Only very small levels, less than 0.2% by weight, of metal halides are required to sufficiently lower the resistivity.
Unfilled urethanes usually show approximately an 18-170 times change in resistivity across environments. In accordance with this invention, this sensitivity was found to be related to the chemical structure of the urethane. Urethanes having caprolactone based polyester moieties have the best environmental sensitivity of any urethane. Their resistivity typically changes by approximately 18-40 times across environments, compared to 40-170 times for other types of poylurethanes which include adipic acid-based polyester urethanes and polyether urethanes. The addition of specific metal halides to the caprolactone-based urethanes reduces this humidity sensitivity to approximately 4-5 times across environments.
In addition, the roller material must have a hardness ranging from 40-60 Shore A, without the use of plasticizer, which can be detrimental to the photoconductor drum material. Also, the roller requires low compression set, less than 5%, to provide uniform printing performance.
In accordance with this invention the polyurethane diisocyanate and metal salt are different from the foregoing prior art to achieve a filled urethane useful as a developmental member which is stable across a wide range of temperature and humidity.